1. Field of the Invention
This invention relates to linear polysiloxanes containing alkylhydrosiloxane units, alkylvinylsiloxane units, and dialkylsiloxane units, which polysiloxanes are siloxy-endblocked. These polysiloxanes are useful as vulcanizable one-component precursors for silicone elastomer compositions. This invention also relates to a process for the preparation of such polysiloxanes, to a process for the conversion of such polysiloxanes to vulcanized silicone elastomers, and to the vulcanizable silicone elastomer compositions so prepared.
2. Description of the Prior Art
Polysiloxanes containing methylhydrosiloxane and methylvinylsiloxane units have been prepared as by-products in the preparation of cyclic siloxanes by hydrolysis of chlorosilanes (K. A. Andrianov, et al., Zhur. Obshch. Khim., 35, 103 (1965); 35 524 (1965); and J. Organometal Chem., 4, 440 (1965)). The polysiloxanes of the above references containing these units were hydroxy-endblocked, i.e. terminated by hydroxyl groups, and water is formed when such siloxanes are cured. Moreover, these polysiloxanes did not contain dialkylsiloxane units in addition to the methylhydrosiloxane and methylvinylsiloxane units. Molecular weight control and reproducibility in preparing these polysiloxanes is difficult due to lack of specific siloxy-endblockers and to the large excess of hydroxy-endblocking reagent, i.e., water, used in such preparations.
U.S. Pat. No. 4,578,493 discloses branched polysiloxanes containing methylhydrosiloxane and methylvinylsiloxane units in combination with other units, including dimethylsiloxane units and trifunctional or tetrafunctional siloxane units, which polysiloxanes are silyl-endblocked to a random and undisclosed extent. Since these polysiloxanes are prepared by the cohydrolysis of Me.sub.3 SiCl, MeSiHCl.sub.2, MeSiCl.sub.2 CH.dbd.CH.sub.2, Me.sub.2 SiCl.sub.2 and HSiCl.sub.3, they are indiscriminately endblocked with Me.sub.3 Si-groups and by hydroxyl groups arising from the hydrolysis preparation.
It is also known that polysiloxanes containing methylhydrosiloxane units are usually equilibrated with an acid catalyst, while polysiloxanes containing methylvinylsiloxane units are usually equilibrated with a base catalyst (The Chemistry and Technology of Silicones, W. Noll, 2nd edition,, (Academic Press: 1968), p.219-229). Further, it does not appear that any single catalyst has been shown to be effective in preparing polysiloxanes containing units of both methylhydrosiloxane and methylvinylsiloxane by equilibration.
U.S. Pat. No. 3,843,702 discloses that trifluoromethanesulfonic acid (CF.sub.3 SO.sub.3 H), commonly referred to as triflic acid, can be a catalyst for the equilibration of lower molecular weight polysiloxanes containing neither silicon to hydrogen bonds nor silicon bonded to a vinyl group. U.S. Pat. Nos. 4,634,610 and 4,634,755 relate to the use of trifluoromethanesulfonic acid to make polysiloxanes having groups such as dimethylsiloxane and methylvinylsiloxane that do not contain silicon to hydrogen bonds. Nowhere do these references disclose an equilibration process for making a polysiloxane containing both silicon bonded to hydrogen and silicon bonded to a vinyl group.
U.S. Pat. No. 4,222,952 discloses that polysiloxanes containing silicon bonded to hydrogen can be rearranged, and, therefore, can be equilibrated, using solid perfluorinated polymers containing pendant sulfonic groups as equilibration catalysts.
U.S. Pat. No. 4,581,391 discloses that methylvinylsiloxane units in polysiloxanes increase the ablation resistance/insulation properties of diorganosiloxane polymers by forming ceramic residue or char on exposure to high temperature. In this reference, the polysiloxanes are cured by the addition of a second polysiloxane containing methylhydrosiloxane units in the presence of a crosslinking platinum catalyst.
U.S. Pat. No. 4,888,376 discloses a hydroxy-endblocked polysiloxane containing methylhydrosiloxane and methylvinylsiloxane units and a siloxy-endblocked polysiloxane having either methylhydrosiloxane units or methylvinylsiloxane units, but not both units in the same polysiloxane. The polysiloxanes of this reference are prepared by hydrolysis.
It is standard practice in the silicones industry to employ two-component vulcanizable systems for preparing silicone elastomers. Typically, one component contains methylvinylsiloxane units and the second component contains methylhydrosiloxane units, with vulcanization occurring through catalyzed addition of methydrosiloxane groups to methylvinylsiloxane groups. This practice continues even in recent patents, such as U.S. Pat. Nos. 4,645,815 and 4,709,001.
Accordingly, the prior art does not disclose linear siloxy-endblocked polysiloxanes containing units of alkylhydrosiloxane, alkylvinylsiloxane, and dialkoxysiloxane that are effective vulcanizable one-component precursors for silicone elastomer compositions. Additionally, the prior art does not disclose an equilibration process for preparing linear siloxy-endblocked polysiloxanes containing units of alkylhydrosiloxane, alkylvinylsiloxane, and dialkylsiloxane.